Referring to FIG. 8, there is shown a heat pump type hot water supply apparatus having a refrigerant cycle 72 and a hot water supply cycle 71. The refrigerant cycle 72 includes a compressor 74, a hot water supply heat exchanger (utilization side heat exchanger) 75, an electric expansion valve 77, and a heat source side heat exchanger (air heat exchanger) 78 which are all connected in the given order. On the other hand, the hot water supply cycle 71 includes a hot water storage tank (hot water supply tank) 70 and a circulating path 79. A water circulation pump 80 and a heat exchanging path 81 are inserted in the circulating path 79. In this case, the heat exchanging path 81 is formed by the utilization side heat exchanger (water heat exchanger) 75.
In the above-described heat pump type hot water supply apparatus, when the compressor 74 is activated while the pump 80 is activated (operated), stored water (warm water) flows out through a water intake opening provided in the bottom of the hot water storage tank 70 and enters the circulating path 79. The warm water then flows in the heat exchanging path 81. At that time, the warm water is heated (boiled up) by the water heat exchanger 75. Then, the heated water passes through a hot water inlet opening of the hot water storage tank 70 and is brought back to a top part of the hot water storage tank 70. In this way, high-temperature warm water is stored in the hot water storage tank 70.
The air heat exchanger 78 functions also as an evaporator. Accordingly, in some cases where the outside air temperature is low, frost formation may occur on the air heat exchanger 78, resulting in a drop in capacity. To cope with this, such a type of heat pump type hot water supply apparatus is able to perform a defrost operation for removing the frost. In other words, a defrost operation of supplying hot gas directly to the air heat exchanger 78 from the compressor 74 is carried out. To this end, for example, a discharge pipe 82 of the compressor 74 and a refrigerant flow path 83 establishing connection between the electric expansion valve 77 and the air heat exchanger 78 are connected together by a defrost circuit 85 provided with a defrost valve 84.
By placing the defrost valve 84 in the open state, hot gas from the compressor 74 is allowed to flow into the defrost circuit 85, and the hot gas is then supplied directly to the air heat exchanger 78 through the defrost circuit 85, whereby the frost on the air heat exchanger 78 is melted and removed. And, during the defrost operation, the water circulation pump 80 of the circulating path 79 is stopped.